Throwout Bearing?

I had a freaky thing happen to me just now. My brother and I were loading my Jeep onto the trailer. It was running to charge the batteries because its been sitting a while. We had just finished tightening down the chain on the front axle when the Jeep started to make this horrific squealing noise coming from the clutch area and it started to smoke. I got the Jeep turned off as fast as I could but there was definitely smoke coming from the clutch. What could have caused it to engage like that while the tranny was in neutral?